Method for producing single crystal of multi-element oxide single crystal containing bismuth as constituting element

ABSTRACT

The present invention provides methods for producing a multi-element oxide single crystal which contains Bi, which has high crystallinity independently of a preparation process, and which is represented by the formula (Bi 2 O 2 )A m−1 B m O 3m+1 , wherein A is Sr, Ba, Ca, or Bi and B is Ti, Ta, or Nb. 
     A flux layer, containing a composition satisfying the inequality 0&lt;CuO/Bi 2 O 3 &lt;2 and/or 0≦TiO/Bi 2 O 3 &lt;7/6 on a molar basis is deposited on a wafer and a single-crystalline thin-film is then deposited on the flux layer placed on the wafer. A melt of a composition which contains raw materials and a flux and which satisfies the above inequality is prepared and the melt is cooled such that a single crystal is grown. A CuO flux layer is deposited on a wafer and Bi—Ti—O is supplied to the flux layer using a Bi 6 Ti 3 O 12 , Bi 7 Ti 3 O 12 , or Bi 8 Ti 3 O 12  target of which the Bi content is greater than that of an object film such that a Bi 4 Ti 3 O 12  single-crystalline thin-film is formed above the wafer.

TECHNICAL FIELD

The present invention relates to methods for producing a thinfilm-shaped or bulk multi-element oxide single crystal which containsbismuth which has high crystallinity.

BACKGROUND ART

A sputtering process, an MBE process, a pulsed laser deposition process,an MOCVD process, or another process is used to produce a thin-film ofsingle-crystalline multi-element oxide, such as Bi₄Ti₃O₁₂, containingbismuth as well known (see, for example, Patent Documents 1 to 6).Furthermore, the following process is used to produce a single crystalas well known: a flux process for growing a single crystal in a solutionusing a flux that is a compound which does not react with a targetsubstance and raw materials and which can be readily separated therefrom(see Patent Document 7). A Bi₂O₃ flux is used to produce a bulk singlecrystal of Bi₄Ti₃O₁₂ by such a flux process (see Non-patent Documents 1and 2).

Fluxes are additives used to produce single crystals. The fluxes promotethe growth of crystals and enable the synthesis of a thermodynamicallyunstable substance by reducing the synthesis temperature. Flux epitaxyis a process for fabricating a thin-film using a flux (see Non-patentDocuments 3 and 4). It has been recently found that a high-quality bulksingle-crystalline thin-film can be fabricated by the flux epitaxy.

The inventors have filed a patent application claiming invention for amethod for producing a single-crystalline oxide thin-film by athree-phase epitaxial process (see Patent Document 8). In this method, aflux layer is formed by depositing a flux material on a wafer in advancesuch that a flux layer is formed on the wafer and a high-qualitythin-film is deposited on the flux layer, the flux material beingBa—Cu—O. The flux material is known in the field of bulk single crystalproduction and provides an element of an NdBa₂Cu₃O₇ single crystal.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 61-6124-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 63-171869 (Japanese Patent No. 2547203)-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 63-171870 (Japanese Patent No. 2547204)-   Patent Document 4: Japanese Unexamined Patent Application    Publication No. 05-246722 (Japanese Patent No. 3195827)-   Patent Document 5: Japanese Unexamined Patent Application    Publication No. 09-67197-   Patent Document 6: Japanese Unexamined Patent Application    Publication No. 10-158094 (Japanese Patent No. 2939530)-   Patent Document 7: Japanese Unexamined Patent Application    Publication No. 10-338599-   Patent Document 8: Japanese Unexamined Patent Application    Publication No. 2002-68893-   Non-patent Document 1: Yoichiro Matsuda, Hiroshi Matsumoto, Akira    Baba, Takashi Goto, and Toshio Hirai, Jpn. J. Appl. Phys., Vol.    31, (1992) 3108-   Non-patent Document 2: Rintaro Aoyagi, Hiroaki Takeda, Soichiro    Okamura, Tadashi Shiosaki, Jpn. J. Appl. Phys., Vol. 40, (2001) 5671-   Non-patent Document 3: K. S. Yun, B. D. Choi, Y. Matsumoto, J. H.    Song, N. Kanda, T. Itoh, M. kawasaki, T. Chikyowl and P. Ahmet, H.    Koinuma, Appl. Phys. Lett. 80, 61-63 (2002)-   Non-patent Document 4: Ryuta Takahashi et. al., “Dai 50 Kai    Ouyoubutsurigaku Kankeirengou Kouenkai Kouenyokoushu”, p. 658,    (March 2003)

DISCLOSURE OF INVENTION

[Problems to be Solved by the Invention]

If a thin-film is deposited on a wafer by the sputtering process, theMBE process, the pulse laser deposition process, or the MOCVD process,the thin-film is apt to lack Bi having a high vapor pressure. If athin-film is prepared in the presence of an excessive amount of Bi asdisclosed in Patent Document 6, this thin-film contains an excessiveamount of Bi and therefore has low crystallinity. A ferroelectric oxide,which is a bismuth layered ferroelectric material such as Bi₄Ti₃O₁₂,Bi₄BaTi₄O₁₅, Bi₄SrTi₄O₁₅, Bi₄CaTi₄O₁₅, SrBi₂Ta₂O₉, or SrBi₂Nb₂O₉, isrepresented by the formula (Bi₂O₂)A_(m−1)B_(m)O_(3m+1), wherein A is Sr,Ba, Ca, or Bi and B is Ti, Ta, or Nb. A single crystal of theferroelectric oxide is prepared using a Bi₂O₃ flux. Since the Bi₂O₃ fluxis harmful to the environment, the amount of the Bi₂O₃ flux used must bereduced.

In order to solve the above problems involved in the known methods, itis an object of the present invention to provide methods for producingan oxide single crystal having high crystallinity independently of apreparation process. The method is useful in producing a single crystalof a multi-element oxide, such as Bi₄Ti₃O₁₂, Bi₄BaTi₄O₁₅, Bi₄SrTi₄O₁₅,Bi₄CaTi₄O₁₅, SrBi₂Ta₂O₉, or SrBi₂Nb₂O₉, containing Bi.

[Means for Solving the Problems]

The present invention provides methods for producing a thin film-shapedor bulk single crystal containing Bi. In order to form such a thinfilm-shaped single crystal above a wafer, a flux layer is deposited onthe wafer in advance and the single crystal is deposited on the fluxlayer placed on the wafer. Alternatively, in order to produce such abulk single crystal, a flux and raw materials for producing the oxidesingle crystal are melted, a crystal nucleus is created by graduallycooling the melt, and the crystal nucleus is grown into the oxide singlecrystal, that is, the oxide single crystal is produced by a flux processso as to have high crystallinity.

(1) A method for producing a multi-element oxide single crystalcontaining bismuth according to present invention includes a step ofdepositing a flux layer, containing a two-element composition satisfyingthe inequality 0<CuO/Bi₂O₃<2 or a three-element composition satisfyingthe inequalities 0<CuO/Bi₂O₃<2 and 0≦TiO/Bi₂O₃<7/6 on a molar basis, ona wafer and a step of depositing a single-crystalline thin-film on theflux layer placed on the wafer.

(2) A method for producing a multi-element oxide single crystalcontaining bismuth according to present invention includes a step ofdepositing a CuO flux layer on a wafer and a step of supplying Bi—Ti—Oto the flux layer to form a Bi₄Ti₃O₁₂ single-crystalline thin-film abovethe wafer using a Bi₆Ti₃O₁₂, Bi₇Ti₃O₁₂, or Bi₈Ti₃O₁₂ target of which theBi content is greater than that of an object film.

(3) In the method specified in Item (1) or (2), the deposition of theflux layer or the single-crystalline thin-film is performed by asputtering process, an MBE process, a pulsed laser deposition process,or an MOCVD process.

(4) In the method specified in any one of Items (1) to (3), the wafer isa SrTiO₃ (001) wafer, an Al₂O₃ wafer, a Si wafer, a LaAlO₃ wafer, a MgOwafer, or a NdGaO₃ wafer.

(5) A method for producing a multi-element oxide single crystalcontaining bismuth according to present invention includes a step ofpreparing a melt of a Bi₂O₃—CuO two-element or Bi₂O₃—CuO—TiOthree-element composition containing raw materials and a flux, thetwo-element composition satisfying the inequality 0<CuO/Bi₂O₃<2, thethree-element composition satisfying the inequalities 0<CuO/Bi₂O₃<2 and0≦TiO/Bi₂O₃<7/6 on a molar basis, and a step of cooling the melt to growa single crystal.

(6) In the method specified in any one of Items (1) to (5), themulti-element oxide single crystal is Bi₄Ti₃O₁₂, Bi₄BaTi₄O₁₅,Bi₄SrTi₄O₁₅, Bi₄CaTi₄O₁₅, SrBi₂Ta₂O₉, or SrBi₂Nb₂O₉.

Since the flux layer containing the two-element composition satisfyingthe inequality 0<CuO/Bi₂O₃<2 or the three-element composition satisfyingthe inequalities 0<CuO/Bi₂O₃<2 and 0≦TiO/Bi₂O₃<7/6 on a molar basis isused and the single-crystalline thin-film is deposited on the flux layerplaced on the wafer, the thin-film has higher crystallinity as comparedto thin-films prepared using a flux containing Bi₂O₃ only or preparedwithout using any flux. This is probably because the flux layercontaining CuO has a high melting point and CuO serves as a catalyst tocontrol the growth of thin-film. Hence, the thin-film has highcrystallinity and high surface flatness on an atomic level. TiO enhancesthe effect of CuO.

In the method including the step of depositing the CuO flux layer on thewafer and the step of supplying Bi—Ti—O to the flux layer to form theBi₄Ti₃O₁₂ single-crystalline thin-film above the wafer using theBi₆Ti₃O₁₂, Bi₇Ti₃O₁₂, or Bi₈Ti₃O₁₂ target of which the Bi content isgreater than that of an object film, an excessive amount of Bi serves asa flux and then finally converted into a Bi—Cu—O flux. This method isuseful in producing a single-crystalline thin-film, made of SrBi₂Ta₂O₉or SrBi₂Nb₂O₉, containing no Ti.

Known thin-films prepared without using any flux have defects ordislocations and the length of the c-axis of the unit cell of the knownthin-films is different from that of bulk single crystals. However, if asingle-crystalline thin-film containing Bi is prepared using a fluxcontaining one of the two- and three-element compositions containing CuOor a flux containing CuO only, the thin-film has neither defects nordislocations but high crystallinity and is flat over a large area.

If the Bi₂O₃—CuO two-element or Bi₂O₃—CuO—TiO three-element compositioncontaining the raw materials and the flux is used to prepare a bulksingle crystal by a flux process, CuO serves as a catalyst during thegrowth of the oxide single crystal to control the crystallinity of theoxide single crystal and prevents the vaporization of Bi having a highvapor pressure.

In a method for producing a single crystal using a flux according to thepresent invention, the composition (proeutectic phase) of a mixture ofraw materials used at the start of the growth of the single crystal iscritical. For the preparation of a thin-film, the mixture forms aproeutectic phase. For the preparation of a bulk single crystal, amixture of a flux and the raw materials forms a proeutectic phase.

Advantages

As described above, according to a method for producing a multi-elementsingle crystal containing Bi, a thin film-shaped or bulk single crystalhaving high crystallinity can be obtained. Since a flux containing aBi₂O₃—CuO two-element or Bi₂O₃—CuO—TiO three-element composition or aflux containing CuO only is used in the method, the single crystalobtained has substantially the same crystallinity as that of singlecrystals prepared using fluxes containing Bi₂O₃ only although a reducedamount of Bi harmful to the environment is used in the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ternary phase diagram of a Bi₂O₃—CuO—TiO three-elementcomposition which contains a flux for producing a thin film-shapedsingle crystal produced by a method according to the present inventionor which contains a flux and raw materials for producing a bulk singlecrystal produced by a method according to the present invention.

FIG. 2 is a schematic view of a deposition system used in a pulsed laserdeposition process used in Example 1 and Comparative Example 1.

FIG. 3 is a schematic view of a deposition system used in a flux processused in Example 2 and Comparative Example 2.

FIG. 4 includes AFM photographs which are alternatives to drawings: theright photograph shows the surface morphology of a single crystalprepared in Example 1 and the left photograph shows the surfacemorphology of a single crystal prepared in Comparative Example 1.

FIG. 5 includes graphs showing XRD patterns of the single crystal ofExample 1 and the single crystal of Comparative Example 1 (the uppergraph shows 2θ-θ XRD patterns and the lower graph shows rocking curvesof (0014) peaks).

FIG. 6 includes optical photographs which are alternatives to drawings:the right photograph shows the appearance of a single crystal preparedin Example 2 and the left photograph shows the appearance of a singlecrystal prepared in Comparative Example 2.

FIG. 7 includes AFM photographs which are alternatives to drawings: theright photograph shows the appearance of a single-crystalline thin-filmprepared in Example 3 and the left photograph shows the appearance of asingle-crystalline thin-film prepared in Comparative Example 3.

FIG. 8 includes TEM photographs which are alternatives to drawings: thelower photograph shows the appearance of the single-crystallinethin-film of Example 3 and the upper photograph shows the appearance ofthe single-crystalline thin-film of Comparative Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a ternary phase diagram of a Bi₂O₃—CuO—TiO three-elementcomposition which contains a flux for producing a thin film-shapedsingle crystal produced by a method according to the present inventionor which contains a flux and raw materials for producing a bulk singlecrystal produced by a method according to the present invention. Thecomposition satisfies the inequalities 0<CuO/Bi₂O₃<2 and 0≦TiO/Bi₂O₃<7/6on a molar basis as shown in FIG. 1. The melting point of Bi₂O₃ is 850°C. and that of CuO is 1200° C. An increase in the content of CuOtherefore increases the melting point of the composition. When the ratioCuO/Bi₂O₃ is less than two, the composition has a melting point of 1000°C. or less and therefore serves as a flux. However, when the ratioCuO/Bi₂O₃ is greater than or equal to two, the composition has a highermelting point and therefore hardly serves as a flux because it isdifficult to melt the composition. The composition more preferablysatisfies the inequality 0<CuO/Bi₂O₃<4/3 on a molar basis.

A multi-element oxide containing bismuth is a ferroelectric materialrepresented by the formula (Bi₂O₂)A_(m−1)B_(m)O_(3m+1), wherein A is Sr,Ba, Ca, or Bi and B is Ti, Ta, or Nb. Since such an oxide is anincongruent compound, the oxide can be precipitated only from a phasecontaining a large amount of Bi. As is clear from a Bi₂O₃—TiO₂ phasediagram, when the ratio TiO/Bi₂O₃ is greater than 7/6, Bi₂Ti₂O₇,Bi₂Ti₄O₁₁, and TiO₂ are precipitated. This is not preferable. Therefore,the composition more preferably satisfies the inequality 0≦TiO/Bi₂O₃<9/8on a molar basis.

Principal steps included in methods of the present invention will now bedescribed. Although a flux layer, a seed layer, and a single-crystallinethin-film can be formed by an ordinary pulse laser deposition (PLD)process, a sputtering process, an MBE process, an MOCVD process, oranother process, the use of the PLD process is described below in detailwith reference to FIG. 2.

Known deposition systems that can be used in the PLD process havevarious structures. Such deposition systems can be used in the methodsof the present invention. One of the deposition systems includes achamber that can be evacuated. A single-crystalline wafer 1 is retainedin the chamber and heated with a heater 2 located on the rear side ofthe wafer 1. Targets 4, 5, and 6 are arranged in the chamber andirradiated with a laser beam emitted from a KrF laser 3 placed outsidethe chamber, whereby the surfaces of the targets are vaporized. Eachtarget used to form a single crystal is a polycrystalline sintered bodyhaving the same composition and structure as those of a ferroelectricoxide to be deposited. Vapor 7 of the target reaches the heated wafer 1,whereby the single crystal is deposited on a flux layer placed on thewafer 1.

The chamber has a gas inlet 8 through which oxygen gas is supplied suchthat the partial pressure of oxygen in the chamber can be adjusted. Inthe PLD process, a layer in process can be oxidized in such a mannerthat oxidation gas such as O₂ or O₃ is introduced into the chambersimultaneously with the start of deposition.

Japanese Unexamined Patent Application Publication Nos. 7-267791 and5-43390 disclose examples of the system. A deposition system disclosedin the former document includes a chamber and an oxygen inlet. A waferis placed on an upper portion of the chamber and a target is placed on alower portion of the chamber such that the target is opposed to thewafer. The oxygen inlet is located close to the wafer. A depositionsystem disclosed in the latter document includes a chamber in whichoxygen gas flows around a wafer in parallel thereto. Examples of thewafer 1 include a SrTiO₃ (001) wafer, a Si wafer, an Al₂O₃ wafer, aLaAlO₃ wafer, a MgO wafer, a NdGaO₃ wafer.

The methods of the present invention can be used to produce an oxidesingle-crystalline material using a flux such as a Bi₂O₃ derivativeincluding Bi₄Ti₃O₁₂, Bi₄BaTi₄O₁₅, Bi₄SrTi₄O₁₅, Bi₄CaTi₄O₁₅, SrBi₂Ta₂O₉,and SrBi₂Nb₂O₉. A method for producing a Bi₄Ti₃O₁₂ single-crystallinethin-film will now be described in detail.

A procedure for depositing a flux layer containing the followingcomposition on a wafer and then depositing the single-crystallinethin-film on the flux layer placed on the wafer is as described below: aBi₂O₃—CuO—TiO three-element composition satisfying the inequalities0<CuO/Bi₂O₃<2and 0≦TiO/Bi₂O₃<7 on a molar basis.

In a first step, one or more targets are selected by remote-controllinga stepping motor from outside. The targets are made of Bi₄Ti₃O₁₂, Bi₂O₃,or CuO. The targets are individually ablated by varying the energy orpulse number of an ablation laser such that a Bi₂O₃—CuO two-elementcomposition satisfying the inequality 0<CuO/Bi₂O₃<2 or the Bi₂O₃—CuO—TiOthree-element composition satisfying the inequalities 0<CuO/Bi₂O₃<2 and0≦TiO/Bi₂O₃<7 can be obtained. If the single target is used, this targetneeds to have the same composition as described above. A seed layerhaving the same composition as that of the single-crystalline thin-filmmay be deposited on the wafer 1 in advance. The first step is preferablyperformed at a wafer temperature of about 400° C. to 600° C. in anoxygen atmosphere having an oxygen partial pressure of about 10 to 400Pa.

In a second step, the temperature of the wafer placed in a chamber usedin the first step is increased. The second step is preferably performedat such a wafer temperature that the flux layer is not vaporized, thatis, a wafer temperature of about 650° C. to 750° C., in an oxygenatmosphere having an oxygen partial pressure of about 10 to 70 Pa.

In a third step, a Bi₄Ti₃O₁₂ vapor is generated from the target made ofBi₄Ti₃O₁₂ and vapor species of an object oxide are deposited on the fluxlayer placed on the wafer, whereby the thin-film having high quality isformed. Since the thin-film is formed at a reduced pressure in anatmosphere having an oxygen partial pressure of about 10 to 70 Pa, theflux layer is substantially liquid under these conditions. After theformation of the thin-film, pieces of the flux layer remain on thethin-film in the form of droplets. These pieces can be removed with anetching solution containing 5% HCl.

In a method for producing an oxide single-crystalline thin-film made ofBi₄Ti₃O₁₂ in such a manner that a CuO flux layer is deposited on a waferand a Bi—Ti—O compound represented by the formula Bi₆Ti₃O_(x) issupplied to the wafer, an excessive amount of Bi serves as a flux and isfinally converted into a Bi—Cu—O flux. This method is preferably used toproduce a single-crystalline thin-film, made of SrBi₂Ta₂O₃ orBi₂Sr₂CaCu₂O₈, containing no Ti.

Steps of this method are as described below.

In a first step, a CuO target is selected and then ablated, whereby aflux layer is formed on a wafer. It is preferable that the partialpressure of oxygen in a chamber be about 800 to 1300 Pa and thetemperature of the wafer be about 750° C. to 850° C. A seed layer may bedeposited on the wafer in advance. Since an increase in the thickness ofthe flux layer made of CuO enhances the crystallinity of thesingle-crystalline thin-film, the flux layer preferably has a thicknessof 10 nm or more and more preferably 20 nm or more.

In a second step, Bi₄Ti₃O₁₂ is deposited on the wafer using a Bi₆Ti₃O₁₂,Bi₇Ti₃O₁₂, or Bi₈Ti₃O₁₂ target of which the Bi content is greater thanthat of an object film. The pressure in the chamber may be the same asthat used in the first step. Since the thin-film is formed at a reducedpressure in an atmosphere having an oxygen partial pressure of about 800to 1300 Pa, the flux layer is substantially liquid under theseconditions. In this method, pieces of the flux layer that remain on thesingle-crystalline thin-film can be removed by ultrasonic cleaning orthe like using a HCl solution.

In a method of the present invention, the sputtering process can be usedfor the vapor deposition process. In the sputtering process, high-energyions produced in plasma are applied to a target and atoms are therebyejected from the target and then deposited on a wafer, whereby acrystalline thin film is formed. In order to control the composition ofthe flux layer made of the Bi₂O₃—CuO two-element composition or theBi₂O₃—CuO—TiO three-element composition, the following technique may beused: a technique in which a single target made of a compositionsatisfying the inequalities 0<CuO/Bi₂O₃<2 and 0≦TiO/Bi₂O₃<7/6 is used ora technique in which the sputtering time of a Ba₄Ti₃O₁₂ target, a Bi₂O₃target, and a CuO target is varied or the time during which the waferpasses over each target is varied.

In a method of the present invention, a chemical vapor deposition (CVD)process can be used for the vapor deposition process. In the chemicalvapor deposition process, organometals having high vapor pressures areprepared and molecules of the organometals are introduced into a chamberfor forming a thin-film using gas, for example, argon. The organometalmolecules are decomposed on a wafer, whereby organic components arescattered and metal atoms are deposited on the wafer. This results inthe formation of the thin-film.

In order to control the flow rate of each organometal, the temperatureof a vessel for vaporizing the organometal is varied or a valve placedin a pipe through the organometal passes is controlled. In thedeposition system having the above configuration, any elements can besupplied by controlling valves placed in pipes for supplying a pluralityof organometals.

In a method of the present invention, the molecular beam epitaxy (MBE)process can be used for the vapor deposition process. In the molecularbeam epitaxy process, elements such as bismuth, titanium, and copper areseparately supplied from effusion cells for generating atomic fluxes.When an element having a high melting point is used, a cell heated withan electron gun may be used. The flow rate of each atomic flux iscontrolled by varying the temperature of the cells and/or the output ofthe electron gun and/or by controlling shutters placed above the cells.

A method for producing a bulk Bi₄Ti₃O₁₂ single crystal will now bedescribed. An apparatus used in a flux process includes ahigh-temperature electric furnace that can be heated to 1500° C. and aplatinum crucible for storing raw materials and a flux. With referenceto FIG. 3, the platinum crucible 11 is placed in an alumina crucible 12such that the platinum crucible 11 is prevented from being distorted. Aspace between the two crucibles is filled with alumina powder 13. Theflux and the raw materials for forming the oxide single crystal aremixed such that the composition of the mixture satisfies theinequalities 0<CuO/Bi₂O₃<2 and 0≦TiO/Bi₂O₃<7/6 on a molar basis. Thatis, the inequalities 0<CuO/Bi₂O₃<2 and 0≦TiO/Bi₂O₃<7/6 on a molar basisis satisfied at the start of the growth of the crystal on a molar basis.

The raw material-flux mixture 14 is placed into the platinum crucible 11and then heated to 1250° C. in the electric furnace. It has beenreported that Bi₄Ti₃O₁₂ is usually melted at 1250° C. and 1 atm. The rawmaterial-flux mixture 14 is maintained at this temperature for 12 hoursand then slowly cooled at a rate of 3 to 12° C./hour. The rawmaterial-flux mixture 14 is cooled to, for example, 900° C. over 120hours, whereby a crystal nucleus is generated and then grown into thecrystal. In this operation, a seed crystal may be used. The resultingraw material-flux mixture 14 is cooled from 900° C. to room temperatureover six hours. In the flux process, the flux remains in the platinumcrucible after the growth of the crystal. The single crystal isseparated from the flux by pouring an acid solution such as a nitricacid solution into the platinum crucible and then taken out of theplatinum crucible. If the flux is not dissolved in the acid solution,the flux can be dissolved in the acid solution by heating the acidsolution, whereby the single crystal can be obtained.

EXAMPLES Example 1

A Bi₄Ti₃O₁₂ ferroelectric thin-film was prepared by an ordinary pulsedlaser deposition process. A target used to deposit an oxide thin-filmfor forming a flux layer was a sintered body (Bi₂O₃:TiO₂:CuO=27:27:2)prepared by processing a mixture of a CuO powder and a powder with a Bito Ti ratio of two to one. A target for forming the Bi₄Ti₃O₁₂ferroelectric thin-film was prepared in such a manner that a Bi₂O₃powder and a TiO₂ powder were compounded at a desired ratio and thecompound was heated at 700° C. in an ordinary electric furnace.

In a first step, a pulsed laser deposition system and a SrTiO₃ (100)wafer were used. Each target was selected with a stepping motorremote-controlled from outside. A Bi₂TiO_(x) sub-layer with a thicknessof 0.9 nm was deposited on the wafer with a pulsed laser and a CuOsub-layer with a thickness of 0.1 nm was deposited on the Bi₂TiO_(x)sub-layer using a CuO target. This operation was repeated 20 times intotal, whereby a flux layer with a thickness of 20 nm was prepared.Since a 1-nm layer portion was formed in one operation, the compositionof the flux layer was uniform in the thickness direction on an atomiclevel. In the first step, the temperature of the wafer was 500° C., thepartial pressure of oxygen was 70 Pa, and the output and pulse frequencyof a KrF excimer laser used were 1.8 J/cm² and 10 Hz, respectively.

In a second step, the wafer temperature was increased to 700° C. over 10minutes and then maintained at 700° C. for 10 minutes.

In a third step, the Bi₄Ti₃O₁₂ thin-film was deposited on the flux layerunder conditions below using the target for forming the Bi₄Ti₃O₁₂thin-film. The thin-film had a thickness of 500 nm. In the third step,the deposition rate was 8 nm/minute, the deposition time was 60 minutes,the wafer temperature was 700° C., the partial pressure of oxygen was 70Pa, and the output and pulse frequency of the KrF excimer laser were 1.8J/cm² and 10 Hz, respectively.

The obtained Bi₄Ti₃O₁₂ thin-film, which was oriented in the c-axis, wasobserved for surface morphology with by AFM and evaluated forcrystallinity by XRD. As shown in the right half of FIG. 4, theobservation by AFM shows that the thin-film has a step/terrace structurehaving a height substantially equal to half of the length of the c-axisof the unit cell and the surface thereof is flat on an atomic level. Asshown in FIG. 5, the evaluation by XRD shows that the (0014) peak ofthis thin-film has a half-width of 0.076 and that of a thin-filmprepared by an ordinary PLD process has a half-width of 0.148, that is,the (0014) peak of this thin-film is very sharp.

Comparative Example 1

A Bi₄Ti₃O₁₂ target was used to form a flux layer similar to that ofExample 1. The target contained no CuO. The flux layer was 20 nm. ABi₄Ti₃O₁₂ thin-film was prepared under the same conditions as those ofExample 1 except the above conditions. As shown in the left half of FIG.4 and FIG. 5, the thin-film has low crystallinity because the flux layercontains neither Ti nor Cu.

Example 2

A raw material powder and a flux powder were weighed out such that amixture of the powders had a weight of 80 g and a Bi₂O₃ to TiO₂ to CuOratio of 10 to 3 to 1 on a molar basis. The mixture was placed into anagate crucible and then agitated. In this operation, ethanol was addedto the mixture such that the powders were uniformly mixed. After ethanolwas removed, the resulting powder mixture was placed into a platinumcrucible (35 ml size). The platinum crucible was placed into an aluminacrucible and a space between the two crucibles was filled with analumina powder.

The alumina crucible was placed into an electric furnace and then heatedto 1250° C. over four hours and maintained at 1250° C. for 12 hours. Thealumina crucible was slowly cooled to 900° C. over 120 hours. Theresulting alumina crucible was cooled from 900° C. to room temperatureover six hours. A single crystal obtained was covered with a flux andcould not be taken out of the alumina crucible; hence, the flux wasremoved with concentrated nitric acid.

The right half of FIG. 6 is an optical photograph, which is analternative to a drawing, showing the appearance of the bulk singlecrystal. Although the powder mixture contained 7% by mole of Cu, whichwas an impurity to Bi₄Ti₃O₁₂, and the flux contained a reduced amount ofBi₂O₃, the bulk single crystal having substantially the same length(about 1 cm) as that of a bulk single crystal disclosed in Non-patentDocument 2 was obtained as shown in the right half of FIG. 6.

Comparative Example 2

A Bi₂O₃ powder and a TiO₂ powder were weighed out such that a mixture ofthese powders had a weight of 80 g and a Bi₂O₃ to TiO₂ ratio of 11 to 3on a molar basis. This mixture contained no CuO but a greater amount ofBi₂O₃ as compared to that of Example 2. This mixture was placed into anagate crucible and then agitated. In this operation, ethanol was addedto this mixture such that these powders were uniformly mixed. Afterethanol was removed, this resulting mixture was placed into a platinumcrucible (35 ml size). The platinum crucible was placed into an aluminacrucible and a space between the two crucibles was filled with analumina powder.

The alumina crucible was placed into an electric furnace and then heatedto 1250° C. over four hours and maintained at 1250° C. for 12 hours. Thealumina crucible was slowly cooled to 900° C. over 120 hours. Theresulting alumina crucible was cooled from 900° C. to room temperatureover six hours. A single crystal obtained was covered with a flux andfixed to the alumina crucible. The flux was removed with concentratednitric acid, whereby the single crystal was obtained.

The left half of FIG. 6 is an optical photograph, which is analternative to a drawing, showing the appearance of the bulk singlecrystal. The bulk single crystal having substantially the same length(about 1 cm) as that of the bulk single crystal disclosed in Non-patentDocument 2 was obtained as shown in the left half of FIG. 6.

Example 3

A Bi₄Ti₃O₁₂ ferroelectric thin-film was prepared by an ordinary pulsedlaser deposition process. A target for forming the Bi₄Ti₃O₁₂ferroelectric thin-film was identical to that used in Example 1. A CuOtarget for forming a CuO flux was a sintered body prepared by processinga CuO powder. A SrTiO₃ (100) wafer having a Bi₂TiO_(x) seed layer,placed thereon, having a thickness of 20 nm was used, the seed layerbeing deposited on the wafer with a KrF excimer laser under thefollowing conditions: a wafer temperature of 800° C., an oxygen partialpressure of 800 Pa, a laser output of 1.8 J/cm², and a pulse frequencyof 10 Hz.

In a first step, a CuO flux layer with a thickness of 20 nm wasdeposited on the seed layer with the KrF excimer laser using the CuOtarget under the following conditions: a wafer temperature of 800° C.,an oxygen partial pressure of 800 Pa, a laser output of 1.8 J/cm², and apulse frequency of 10 Hz.

In a second step, the thin-film was deposited on the CuO flux layer withthe KrF excimer laser using the target having a composition representedby the formula Bi₆Ti₃O_(x) under the following conditions: a wafertemperature of 800° C., an oxygen partial pressure of 800 Pa, a laseroutput of 1.8 J/cm², a pulse frequency of 10 Hz, a deposition rate of 8nm/minute, and a deposition time of 60 minutes. An amount of BiO thatwas excessive to the Bi₄Ti₃O₁₂ thin-film was converted into a CuO—BiOflux, which was precipitated on the surface of the film, whereby theBi₄Ti₃O₁₂ thin-film was formed. The Bi₄Ti₃O₁₂ thin-film had a thicknessof 300 nm. The flux remaining on the Bi₄Ti₃O₁₂ thin-film was removed byultrasonically cleaning the Bi₄Ti₃O₁₂ thin-film for two seconds using asolution containing 5% by volume of HCl. The obtained Bi₄Ti₃O₁₂thin-film had a large grain size of several microns.

Comparative Example 3

A Bi₄Ti₃O₁₂ thin-film was prepared under the same conditions as thosedescribed in Example 1 except that no CuO flux was used. The right halfof FIG. 7 is an AFM image of the Bi₄Ti₃O₁₂ thin-film prepared in Example3 and the left half thereof is an AFM image of the Bi₄Ti₃O₁₂ thin-filmprepared in Comparative Example 3. In the left half of FIG. 7 that showsthe Bi₄Ti₃O₁₂ thin-film prepared using no CuO flux at highmagnification, dislocations due to a step wafer are observed. Incontrast, in the right half of FIG. 7 that shows the Bi₄Ti₃O₁₂ thin-filmprepared using the CuO flux, no dislocations are observed, that is, thisBi₄Ti₃O₁₂ thin-film is flat over a large area (5 micron square) on anatomic level. The same result as that observed by AFM can be obtained byTEM. In the upper half of FIG. 8 that shows the Bi₄Ti₃O₁₂ thin-filmprepared in Comparative Example 3, dislocations due to a step wafer areobserved. However, in the lower half of FIG. 8 that shows the Bi₄Ti₃O₁₂thin-film prepared in Example 3, no dislocations are observed.

INDUSTRIAL APPLICABILITY

A method of the present invention is useful in producing a defect-freethin-film material or bulk material made of single-crystallineBi₄Ti₃O₁₂. These materials are expected to be applied to wafers forthin-film deposition and/or lead-free non-volatile ferroelectricmemories.

1. A method for producing a multi-element oxide single crystalcontaining bismuth, comprising: a step of depositing a flux layer on awafer, said flux layer containing a three-element composition satisfyingthe inequalities 0<CuO/Bi₂O₃<2 and 0<TiO/Bi₂O₃<7/6 on a molar basis; anda step of depositing a single-crystalline thin-film on the flux layerplaced on the wafer.
 2. A method for producing a multi-element oxidesingle crystal containing bismuth, comprising: a step of depositing aflux layer consisting of CuO on a wafer; and a step of supplying Bi—Ti—Oto the flux layer to form a Bi₄Ti₃O₁₂ single-crystalline thin-film abovethe wafer using a Bi₆Ti₃O₁₂, Bi₇Ti₃O₁₂, or Bi₈Ti₃O₁₂ target of which theBi content is greater than that of a object film.
 3. The methodaccording to claim 1 or 2, wherein the deposition of the flux layer orthe single-crystalline thin-film is performed by a sputtering process,an MBE process, a pulsed laser deposition process, or an MOCVD process.4. The method according to claim 1 or 2, wherein the wafer is a SrTiO₃(001) wafer, an Al₂O₃ wafer, a Si wafer, a LaAlO₃ wafer, a MgO wafer, ora NdGaO₃ wafer.
 5. A method for producing a multi-element oxide singlecrystal, comprising: a step of depositing a flux layer on a wafer, saidflux layer containing a three-element composition satisfying theinequalities 0<CuO/Bi₂O₃<2 and 0<TiO/Bi₂O₃<7/6 on a molar basis, and astep of depositing a single-crystalline thin-film on the flux layerplaced on the wafer, wherein the single-crystalline thin-film comprisesa multi-element oxide selected from the group consisting of Bi₄Ti₃O₁₂,Bi₄BaTi₄O₁₅, Bi₄SrTi₄O₁₅, Bi₄CaTi₄O₁₅, SrBi₂Ta₂O₉, and SrBi₂Nb₂O₉.